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Velocity field measurements of flow around a triangular prism behind a porous fence
Journal of Wind Engineering and Industrial Aerodynamics 77&78 (1998) 521—530
Velocity field measurements of flow around a triangular prism behind a porous fence Sang-Joon Lee*, Hyoung-Bum Kim Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang 790-784, South Korea
Abstract The shelter effect of a porous wind fence on a triangular prism was experimentally simulated in a circulating water channel. A porous wind fence of porosity e"38.5% was installed in front of the prism model. The fence and prism model were embedded in a turbulent boundary layer at Reynolds number of about 3460. The instantaneous velocity field around the fence and prism model was measured using a two-frame Particle Tracking Velocimetry (PTV) technique. In addition to the well-known mean velocity reduction, the fence decreases the turbulent intensity and turbulent kinetic energy around the prism. Especially, at the top of the prism model, the turbulent kinetic energy is about half of that without the fence. By installing the fence in front of the prism, the length of the re-circulation region also decreases compared with that of the no fence case. 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction Wind-blown dust from coal piles may cause economic losses and air-pollution problems. This kind of wind erosion problem can be found in open storage coal yards of electric power stations, mine fields and iron works. Several natural or artificial barriers have been often used around the coal piles to abate the wind-blown dust problem. However, a surface-mounted vertical fence causes complex flow characteristics such as high shear rate, large pressure gradient and high turbulent intensity in the near wake behind the fence. Porous wind fences have been studied by many researchers as an artificial barrier to reduce the oncoming wind speed. Previous studies on a porous wind fence put emphasis on the mean velocity reduction and the turbulent momentum of the wind
* Corresponding author. 0167-6105/98/$ — see front matter 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 7 - 6 1 0 5 ( 9 8 ) 0 0 1 6 9 - X
522
S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
fence wake. Earlier researches found that the fence porosity was the most influential parameter determining the fence wake. Raine and Stevenson [1] measured the wake flow behind porous wind fences using a hot-wire anemometer. They classified the wake flow behind the fence into two regions; the bleed flow dominant region and the displacement flow dominant region. Ranga et al. [2] found that the separation bubbles formed behind the fence disappeared by increasing the geometric porosity in excess of 30%. They also revealed two peaks of turbulent intensity in the near-wake region. Perera [3] investigated the flow characteristics of wakes behind various porous wind fences using a pulsed wire anemometry (PWA). He found that the stagnation point was retarded by increasing the fence porosity and the PWA resolved the problems with hot-wire anemometry for measuring separated turbulent wake flows. Despite its practical importance, the flow around coal piles behind a porous wind fence has received relatively little attention due to its complex flow structure. It was known that the wind erosion phenomena were closely related to the flow characteristics near the surface of the coal pile, especially the mean velocity and the wall shear stress [4]. Borges and Viegas [5] studied the shelter effect of porous fences on the coal pile by measuring the velocity defect and the shear stress distribution. They found that the surface shear stress decreased as the ratio of the fence height to the crest height of the coal pile increased. Since most previous studies employed point-wise measurement techniques such as hot-wire, PWA and LDV system, the results consisted of time-averaged velocities and turbulent intensities at discrete points. Recently, due to the development of computers, optics and digital image processing techniques, instantaneous velocity fields can be extracted by special techniques such as particle image velocimetry (PIV) and particle tracking velocimetry (PTV). In this study, the flow fields around the porous wind fence and the triangular prism were investigated experimentally using the two-frame PTV method [6,7] to understand the shelter effect of porous wind fence on the coal pile model. With a better understanding of the flow characteristics around the fence and prism, the problem of wind-blown dust from coal piles may be solved successfully.
2. Experimental apparatus and methods The experiment was carried out in a circulating water channel of which the test section size was 300 (w);200 (h);1200 (l) (mm). A schematic diagram of the fence and prism model with the coordinate system used in this study is shown in Fig. 1. The fence model was made of stainless steel of 0.3 mm thickness (B). The fence height (H) was 25 mm and had a flat end. Since the shape ratio (B/H"0.012) is less than 0.33, the flow can be regarded as a fence flow. The porous fence had a geometric porosity (open area percentage) e"38.5% consisting of uniformly distributed circular holes. From preliminary tests, this porosity of the fence was found to be good for velocity and turbulent momentum reduction. The fence model was installed 0.8 m behind the inlet of the test section. A triangular prism having an inclination angle of 40° was located at
S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
523
X"1H behind the wind fence. The crest height (h) of the prism was 17 mm. During the experiments, the free-stream velocity was maintained at 12.5 cm/s and its corresponding Reynolds number (Re) based on the fence height was about 3460. Local Reynolds number (Re ) based on the fence location was 1.11;10. A trip wire was installed at the inlet of the test section to make the fence and prism fully submerged in a turbulent boundary layer. Turbulence Reynolds number based on the momentum thickness was about 750. The two-frame PTV measurement system developed for this study consisted of a laser light sheet, CCD camera, frame grabber and computer. A 4 W Ar-ion laser provides a light sheet, which had approximately 3 mm thickness by passing through a fiber-optical cable and a cylindrical lens. Polystyrene particles with a mean diameter of 150 lm were seeded into the working flow before the experiment. Velocity field measurements were made at three consecutive cross-sections along the central plane of the water channel. The overall measurement area was about !1H)X)10H as shown in Fig. 1. A flow region of about 10 mm was overlapped between two measurement sections. The flow images were captured by an interline transfer CCD camera (Panasonic WV-BL600) with a resolution of 682;492 pixels. The flow images were digitized by a frame grabber (DT-2862) in an array of 512;480 pixels, each having 256 gray levels. The digitized particle images were transferred to an engineering workstation and then split into even and odd fields. Noise removing, image enhancement, extraction and labeling of particles were performed in the preprocessing routine. Information on the particle centroid was given as input data to the two-frame particle tracking algorithm in order to obtain the correct velocity vectors at each particle location. For more details of the algorithm and its accuracy, refer to Refs. [6,7]. Since the two-frame PTV technique requires only two consecutive images and each frame captured by the CCD camera can be divided into even and odd fields, an instantaneous velocity vector field can be obtained at every 1/60 s. The instantaneous velocity vectors at random particle locations were interpolated into regularly spaced grid points using an adaptive Gaussian window. In this experiment, a total of 350
Fig. 1. Schematic diagram of fence and prism model with a coordinate system.
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S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
instantaneous velocity vector fields were acquired at each measurement section. The mean velocity field was obtained by ensemble averaging the 350 instantaneous velocity vector fields on the grid points (43;42 grids). Since the PTV technique implemented in this study measured only two velocity components, we made the fence model to have a large aspect ratio in order to get two-dimensional flow characteristics at the central section. Preliminary flow visualization showed that the lateral flow motion was very weak. The fluctuating velocity vector fields were obtained by subtracting the mean velocity field from the instantaneous velocity vector fields. All the fluctuating velocity vector fields were statistically averaged to get the turbulence properties such as the turbulent intensity. The spanwise component of instantaneous vorticity was calculated using the following equation;
1 *v *u 1 v !v u !u G\ H! G H> G H\ ! + G> H u" X 2 *x *y 2 2*y 2*x
The turbulent kinetic energy was calculated from the following two-dimensional approximation; w"(u#v), ¹ "o(u#v#w)"o(u#v) This assumption uses the concept of isotropic flow structure. Therefore, the real turbulent kinetic energy will be a little different from the present result at the regions near the fence top and prism crest.
3. Results and discussions The mean velocity vector fields around a triangular prism model with and without a porous fence installed are shown in Fig. 2. Without the wind fence, the oncoming flow is accelerated along the windward side of the prism and separated near the prism crest. When a wind fence with porosity e"38.5% is installed in front of the prism, from the shelter effect of the fence the mean velocity is largely reduced between the fence and the prism model. In addition, the approaching flow is separated at the top edge of the fence and the upper part of the prism model is located below the separated shear layer developed from the fence. Fig. 3 shows the cross-sectional velocity profiles of the mean streamwise and vertical velocity components at several downstream locations. The effect of the porous wind fence on the reduction of the mean velocity is eminent in the region between the fence and the prism. With the porous wind fence, the streamwise velocities have much smaller values in the lower part of this region, compared with those of the no fence case. For example, at X/H"1 location, the mean velocity at the fence height is only 37% of the no fence case. From this we can see that the shelter effect of the fence leads to a reduction of the mean velocity in the lower wake region behind the wind fence. However, the difference in the mean streamwise velocity profiles at downstream
S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
525
Fig. 2. Mean velocity vector field around the triangular prism.
locations behind the re-circulation region is not appreciable whether the porous wind fence is present or not. In the case of vertical velocity profiles, the effect of the porous fence is most dominant just above the prism crest at x/H"1. By installing the wind fence, the vertical velocity at this location decreases more than half of that without the fence. Fig. 4 shows the interpolated fluctuating velocity vector fields with fence and without fence. Without fence, the flow around the fence shows peculiar flow characteristics, which can be seen in the wake flow behind a sharp-edged bluff body located in an atmospheric boundary layer. The oncoming flow separates at the prism crest and with the re-circulating flow ascending reversibly along the leeward side of prism. Therefore, we can see large-scale vortices shed from the top of the prism in the shear layer behind the leeward side of the prism. By installing the wind fence, the level of velocity fluctuations associated with these vortices is largely reduced, especially near the prism surface, compared with those for which the fence is absent. In addition, almost all of the re-circulating bubbles and large vortices which can be seen for the no fence case have disappeared behind the prism model. This will decrease the windblown dust erosion from the model surface. But, the wind fence separates the
526
S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
Fig. 3. Comparison of mean streamwise (a) and vertical (b) velocity.
Fig. 4. Instantaneous fluctuation velocity vector field.
oncoming flow at its top and causes the formation of a turbulent shear layer above the fence height of the wake region. Velocity fluctuations in this shear layer are much stronger than that without the fence. From this we can see that the fence height has to be higher than that of the prism crest. If the fence is lower than the height of the coal
S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
527
piles, the upper side of the coal pile will be located inside of a strong mixing layer with high velocity fluctuations. Fig. 5 shows the turbulent intensity profiles of the streamwise and vertical velocity components. These are extracted from ensemble averaging the 350 instantaneous fluctuation vector fields at corresponding grid points. When a porous wind fence is located in front of the prism, the fence wake has three times larger turbulent intensity values in the turbulent shear layer compared with the outer flow region, which is formed from the top of the fence. The streamwise turbulent intensity profiles show this phenomenon quite well, we can see a sharp peak just behind the fence and the local maximum of the turbulent intensity moves upward with going downstream. The width of the shear layer is broader compared to that without the fence. However, in the region below the fence height (y/H"1), the magnitudes of the streamwise and vertical turbulent intensities are smaller than those of the no fence case. This indicates that the porous fence not only decreases the mean velocity but also suppresses the velocity fluctuations in the lower part of the wake region. Without the fence, the effect of the prism can be seen clearly from the streamwise turbulent intensity profiles at
Fig. 5. Cross-sectional profiles of turbulent intensity of streamwise (a) and vertical (b) velocity component.
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S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
Fig. 6. Comparison of turbulent kinetic energy.
X"(2—4)H locations. The shape of the vertical turbulent intensity profile is nearly similar to the streamwise turbulent intensity at downstream behind X"7H location. However, the maximum vertical turbulent intensity is about half of the streamwise turbulent intensity. In addition, it is difficult to find the second peak of the streamwise turbulent intensity behind the fence as reported by Ranga et al. [2]. Comparison results of the turbulent kinetic energy profiles with and without the wind fence are shown in Fig. 6. The general shape is similar to the streamwise turbulent intensity. This results from the fact that the vertical turbulent intensity is about half of the streamwise turbulent intensity. Due to the existence of the fence, the turbulent kinetic energy has higher values above the fence height. On the other hand, the turbulent kinetic energy has much smaller values in the wake region below the fence height. Especially, at the top of the prism model, the porous wind fence reduces the turbulent kinetic energy up to 1/3 of that without the fence. Vorticity contour plots for both cases are compared in Fig. 7. A clockwise rotating, negative vorticity is formed behind the triangular prism model in both cases. When a fence is installed in front of the prism, the size and strength of the positive vorticity is smaller than that without the prism. And we can see clockwise rotating vortices located between the fence and prism. The strength of the vorticity in front of the prism is a little stronger than that developed behind the prism.
4. Conclusion The velocity field of the flow around the wind fence and the triangular prism was measured by the two-frame PTV technique in a circulating water tunnel to investigate the effect of an upstream porous fence on the flow structure around the prism model. A porous wind fence of porosity e"38.5% was installed in front of the prism model and both of them were embedded in a turbulent boundary layer at Reynolds number of about 3460.
S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
529
Fig. 7. Vorticity contour plot of triangular prism with fence and without fence.
By installing the fence (height H"1.47h) in front of the prism, the prism model is located below the shear layer developed from top of the fence. Due to the shelter effect of the fence, the reduction of mean velocity and velocity fluctuations is dominant in front of the prism model. The size of the re-circulation region and the strength of vorticity formed behind the prism are smaller compared with those of the no fence case. In the wake region below the fence height, the turbulent intensity and the turbulent kinetic energy are greately decreased around the prism. Especially, at the top of the prism model, the turbulent kinetic energy is about half of that without the fence. The porous fence used in this study could be effective for reducing the wind erosion by decreasing the velocity and turbulent momentum transfer around the prism surface. But, the fence height should be higher than that of the prism crest.
References [1] J.K. Raine, D.C. Stevenson, Wind protection by model fences in a simulated atmospheric boundary layer, J. Ind. Aerodyn. 2 (1977) 159—180.
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S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
[2] R.K.G. Ranga, R.J. Garde, S.K. Singh, N. Singh, Experimental study on characteristics of flow past porous fences, J. Wind Eng. Ind. Aerodyn. 29 (1988) 155—163. [3] M.D.A.E.S. Perera, Shelter behind two-dimensional solid and porous fences, J. Wind Eng. Ind. Aerodyn. 8 (1981) 93—104. [4] J. Xuan, W. Ye, Wind tunnel modeling of dust emission and deposition in lower atmosphere: similarity principles, Proc. 3rd Asia—Pacific Symp. on Wind Engineering, 1993, pp. 1053—1058. [5] A.R. Borges, D.X. Viegas, Shelter effect on a row of coal piles to prevent wind erosion, J. Wind Eng. Ind. Aerodyn. 29 (1988) 145—154. [6] S.J. Lee, S.J. Baek, Two-frame PTV and its application to a turbulent channel flow, Adv. Turbulent Res. (1995) 21—42. [7] S.J. Lee, S.J. Baek, A new two-frame particle tracking algorithm using match probability, Exp. Fluids 22 (1996) 23—32.
Velocity field measurements of flow around a triangular prism behind a porous fence Sang-Joon Lee*, Hyoung-Bum Kim Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang 790-784, South Korea
Abstract The shelter effect of a porous wind fence on a triangular prism was experimentally simulated in a circulating water channel. A porous wind fence of porosity e"38.5% was installed in front of the prism model. The fence and prism model were embedded in a turbulent boundary layer at Reynolds number of about 3460. The instantaneous velocity field around the fence and prism model was measured using a two-frame Particle Tracking Velocimetry (PTV) technique. In addition to the well-known mean velocity reduction, the fence decreases the turbulent intensity and turbulent kinetic energy around the prism. Especially, at the top of the prism model, the turbulent kinetic energy is about half of that without the fence. By installing the fence in front of the prism, the length of the re-circulation region also decreases compared with that of the no fence case. 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction Wind-blown dust from coal piles may cause economic losses and air-pollution problems. This kind of wind erosion problem can be found in open storage coal yards of electric power stations, mine fields and iron works. Several natural or artificial barriers have been often used around the coal piles to abate the wind-blown dust problem. However, a surface-mounted vertical fence causes complex flow characteristics such as high shear rate, large pressure gradient and high turbulent intensity in the near wake behind the fence. Porous wind fences have been studied by many researchers as an artificial barrier to reduce the oncoming wind speed. Previous studies on a porous wind fence put emphasis on the mean velocity reduction and the turbulent momentum of the wind
* Corresponding author. 0167-6105/98/$ — see front matter 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 7 - 6 1 0 5 ( 9 8 ) 0 0 1 6 9 - X
522
S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
fence wake. Earlier researches found that the fence porosity was the most influential parameter determining the fence wake. Raine and Stevenson [1] measured the wake flow behind porous wind fences using a hot-wire anemometer. They classified the wake flow behind the fence into two regions; the bleed flow dominant region and the displacement flow dominant region. Ranga et al. [2] found that the separation bubbles formed behind the fence disappeared by increasing the geometric porosity in excess of 30%. They also revealed two peaks of turbulent intensity in the near-wake region. Perera [3] investigated the flow characteristics of wakes behind various porous wind fences using a pulsed wire anemometry (PWA). He found that the stagnation point was retarded by increasing the fence porosity and the PWA resolved the problems with hot-wire anemometry for measuring separated turbulent wake flows. Despite its practical importance, the flow around coal piles behind a porous wind fence has received relatively little attention due to its complex flow structure. It was known that the wind erosion phenomena were closely related to the flow characteristics near the surface of the coal pile, especially the mean velocity and the wall shear stress [4]. Borges and Viegas [5] studied the shelter effect of porous fences on the coal pile by measuring the velocity defect and the shear stress distribution. They found that the surface shear stress decreased as the ratio of the fence height to the crest height of the coal pile increased. Since most previous studies employed point-wise measurement techniques such as hot-wire, PWA and LDV system, the results consisted of time-averaged velocities and turbulent intensities at discrete points. Recently, due to the development of computers, optics and digital image processing techniques, instantaneous velocity fields can be extracted by special techniques such as particle image velocimetry (PIV) and particle tracking velocimetry (PTV). In this study, the flow fields around the porous wind fence and the triangular prism were investigated experimentally using the two-frame PTV method [6,7] to understand the shelter effect of porous wind fence on the coal pile model. With a better understanding of the flow characteristics around the fence and prism, the problem of wind-blown dust from coal piles may be solved successfully.
2. Experimental apparatus and methods The experiment was carried out in a circulating water channel of which the test section size was 300 (w);200 (h);1200 (l) (mm). A schematic diagram of the fence and prism model with the coordinate system used in this study is shown in Fig. 1. The fence model was made of stainless steel of 0.3 mm thickness (B). The fence height (H) was 25 mm and had a flat end. Since the shape ratio (B/H"0.012) is less than 0.33, the flow can be regarded as a fence flow. The porous fence had a geometric porosity (open area percentage) e"38.5% consisting of uniformly distributed circular holes. From preliminary tests, this porosity of the fence was found to be good for velocity and turbulent momentum reduction. The fence model was installed 0.8 m behind the inlet of the test section. A triangular prism having an inclination angle of 40° was located at
S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
523
X"1H behind the wind fence. The crest height (h) of the prism was 17 mm. During the experiments, the free-stream velocity was maintained at 12.5 cm/s and its corresponding Reynolds number (Re) based on the fence height was about 3460. Local Reynolds number (Re ) based on the fence location was 1.11;10. A trip wire was installed at the inlet of the test section to make the fence and prism fully submerged in a turbulent boundary layer. Turbulence Reynolds number based on the momentum thickness was about 750. The two-frame PTV measurement system developed for this study consisted of a laser light sheet, CCD camera, frame grabber and computer. A 4 W Ar-ion laser provides a light sheet, which had approximately 3 mm thickness by passing through a fiber-optical cable and a cylindrical lens. Polystyrene particles with a mean diameter of 150 lm were seeded into the working flow before the experiment. Velocity field measurements were made at three consecutive cross-sections along the central plane of the water channel. The overall measurement area was about !1H)X)10H as shown in Fig. 1. A flow region of about 10 mm was overlapped between two measurement sections. The flow images were captured by an interline transfer CCD camera (Panasonic WV-BL600) with a resolution of 682;492 pixels. The flow images were digitized by a frame grabber (DT-2862) in an array of 512;480 pixels, each having 256 gray levels. The digitized particle images were transferred to an engineering workstation and then split into even and odd fields. Noise removing, image enhancement, extraction and labeling of particles were performed in the preprocessing routine. Information on the particle centroid was given as input data to the two-frame particle tracking algorithm in order to obtain the correct velocity vectors at each particle location. For more details of the algorithm and its accuracy, refer to Refs. [6,7]. Since the two-frame PTV technique requires only two consecutive images and each frame captured by the CCD camera can be divided into even and odd fields, an instantaneous velocity vector field can be obtained at every 1/60 s. The instantaneous velocity vectors at random particle locations were interpolated into regularly spaced grid points using an adaptive Gaussian window. In this experiment, a total of 350
Fig. 1. Schematic diagram of fence and prism model with a coordinate system.
524
S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
instantaneous velocity vector fields were acquired at each measurement section. The mean velocity field was obtained by ensemble averaging the 350 instantaneous velocity vector fields on the grid points (43;42 grids). Since the PTV technique implemented in this study measured only two velocity components, we made the fence model to have a large aspect ratio in order to get two-dimensional flow characteristics at the central section. Preliminary flow visualization showed that the lateral flow motion was very weak. The fluctuating velocity vector fields were obtained by subtracting the mean velocity field from the instantaneous velocity vector fields. All the fluctuating velocity vector fields were statistically averaged to get the turbulence properties such as the turbulent intensity. The spanwise component of instantaneous vorticity was calculated using the following equation;
1 *v *u 1 v !v u !u G\ H! G H> G H\ ! + G> H u" X 2 *x *y 2 2*y 2*x
The turbulent kinetic energy was calculated from the following two-dimensional approximation; w"(u#v), ¹ "o(u#v#w)"o(u#v) This assumption uses the concept of isotropic flow structure. Therefore, the real turbulent kinetic energy will be a little different from the present result at the regions near the fence top and prism crest.
3. Results and discussions The mean velocity vector fields around a triangular prism model with and without a porous fence installed are shown in Fig. 2. Without the wind fence, the oncoming flow is accelerated along the windward side of the prism and separated near the prism crest. When a wind fence with porosity e"38.5% is installed in front of the prism, from the shelter effect of the fence the mean velocity is largely reduced between the fence and the prism model. In addition, the approaching flow is separated at the top edge of the fence and the upper part of the prism model is located below the separated shear layer developed from the fence. Fig. 3 shows the cross-sectional velocity profiles of the mean streamwise and vertical velocity components at several downstream locations. The effect of the porous wind fence on the reduction of the mean velocity is eminent in the region between the fence and the prism. With the porous wind fence, the streamwise velocities have much smaller values in the lower part of this region, compared with those of the no fence case. For example, at X/H"1 location, the mean velocity at the fence height is only 37% of the no fence case. From this we can see that the shelter effect of the fence leads to a reduction of the mean velocity in the lower wake region behind the wind fence. However, the difference in the mean streamwise velocity profiles at downstream
S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
525
Fig. 2. Mean velocity vector field around the triangular prism.
locations behind the re-circulation region is not appreciable whether the porous wind fence is present or not. In the case of vertical velocity profiles, the effect of the porous fence is most dominant just above the prism crest at x/H"1. By installing the wind fence, the vertical velocity at this location decreases more than half of that without the fence. Fig. 4 shows the interpolated fluctuating velocity vector fields with fence and without fence. Without fence, the flow around the fence shows peculiar flow characteristics, which can be seen in the wake flow behind a sharp-edged bluff body located in an atmospheric boundary layer. The oncoming flow separates at the prism crest and with the re-circulating flow ascending reversibly along the leeward side of prism. Therefore, we can see large-scale vortices shed from the top of the prism in the shear layer behind the leeward side of the prism. By installing the wind fence, the level of velocity fluctuations associated with these vortices is largely reduced, especially near the prism surface, compared with those for which the fence is absent. In addition, almost all of the re-circulating bubbles and large vortices which can be seen for the no fence case have disappeared behind the prism model. This will decrease the windblown dust erosion from the model surface. But, the wind fence separates the
526
S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
Fig. 3. Comparison of mean streamwise (a) and vertical (b) velocity.
Fig. 4. Instantaneous fluctuation velocity vector field.
oncoming flow at its top and causes the formation of a turbulent shear layer above the fence height of the wake region. Velocity fluctuations in this shear layer are much stronger than that without the fence. From this we can see that the fence height has to be higher than that of the prism crest. If the fence is lower than the height of the coal
S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
527
piles, the upper side of the coal pile will be located inside of a strong mixing layer with high velocity fluctuations. Fig. 5 shows the turbulent intensity profiles of the streamwise and vertical velocity components. These are extracted from ensemble averaging the 350 instantaneous fluctuation vector fields at corresponding grid points. When a porous wind fence is located in front of the prism, the fence wake has three times larger turbulent intensity values in the turbulent shear layer compared with the outer flow region, which is formed from the top of the fence. The streamwise turbulent intensity profiles show this phenomenon quite well, we can see a sharp peak just behind the fence and the local maximum of the turbulent intensity moves upward with going downstream. The width of the shear layer is broader compared to that without the fence. However, in the region below the fence height (y/H"1), the magnitudes of the streamwise and vertical turbulent intensities are smaller than those of the no fence case. This indicates that the porous fence not only decreases the mean velocity but also suppresses the velocity fluctuations in the lower part of the wake region. Without the fence, the effect of the prism can be seen clearly from the streamwise turbulent intensity profiles at
Fig. 5. Cross-sectional profiles of turbulent intensity of streamwise (a) and vertical (b) velocity component.
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S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
Fig. 6. Comparison of turbulent kinetic energy.
X"(2—4)H locations. The shape of the vertical turbulent intensity profile is nearly similar to the streamwise turbulent intensity at downstream behind X"7H location. However, the maximum vertical turbulent intensity is about half of the streamwise turbulent intensity. In addition, it is difficult to find the second peak of the streamwise turbulent intensity behind the fence as reported by Ranga et al. [2]. Comparison results of the turbulent kinetic energy profiles with and without the wind fence are shown in Fig. 6. The general shape is similar to the streamwise turbulent intensity. This results from the fact that the vertical turbulent intensity is about half of the streamwise turbulent intensity. Due to the existence of the fence, the turbulent kinetic energy has higher values above the fence height. On the other hand, the turbulent kinetic energy has much smaller values in the wake region below the fence height. Especially, at the top of the prism model, the porous wind fence reduces the turbulent kinetic energy up to 1/3 of that without the fence. Vorticity contour plots for both cases are compared in Fig. 7. A clockwise rotating, negative vorticity is formed behind the triangular prism model in both cases. When a fence is installed in front of the prism, the size and strength of the positive vorticity is smaller than that without the prism. And we can see clockwise rotating vortices located between the fence and prism. The strength of the vorticity in front of the prism is a little stronger than that developed behind the prism.
4. Conclusion The velocity field of the flow around the wind fence and the triangular prism was measured by the two-frame PTV technique in a circulating water tunnel to investigate the effect of an upstream porous fence on the flow structure around the prism model. A porous wind fence of porosity e"38.5% was installed in front of the prism model and both of them were embedded in a turbulent boundary layer at Reynolds number of about 3460.
S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
529
Fig. 7. Vorticity contour plot of triangular prism with fence and without fence.
By installing the fence (height H"1.47h) in front of the prism, the prism model is located below the shear layer developed from top of the fence. Due to the shelter effect of the fence, the reduction of mean velocity and velocity fluctuations is dominant in front of the prism model. The size of the re-circulation region and the strength of vorticity formed behind the prism are smaller compared with those of the no fence case. In the wake region below the fence height, the turbulent intensity and the turbulent kinetic energy are greately decreased around the prism. Especially, at the top of the prism model, the turbulent kinetic energy is about half of that without the fence. The porous fence used in this study could be effective for reducing the wind erosion by decreasing the velocity and turbulent momentum transfer around the prism surface. But, the fence height should be higher than that of the prism crest.
References [1] J.K. Raine, D.C. Stevenson, Wind protection by model fences in a simulated atmospheric boundary layer, J. Ind. Aerodyn. 2 (1977) 159—180.
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S.-J. Lee, H.-B. Kim/J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 521–530
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